Method and system for improved state estimation accuracy in battery management systems with cell balancing

Abstract

When balancing cells in battery packs, the voltage induced by the balancing current across the parasitic wire resistance distorts the measured cell voltage, leading to inaccurate cell state estimation. A method and battery management system to extract a parasitic resistance of battery connection wires connecting a cell of a battery to a cell balancing circuit is disclosed. The parasitic wire resistances for a battery pack are extracted through the coordinated operation of a pack-level current source and cell-level balancing circuit. A voltage drop is imposed and measured across the parasitic resistances while maintaining zero current in the immediate battery cell. Maintaining zero current in the immediate battery cell avoids possible extraction error resulting from voltage drop across the battery cell impedance due to battery cell current flow.

The invention claimed is: 
     
       1. A method of extracting a parasitic resistance of battery connection wires in a battery, wherein the battery comprises a plurality of cells, each respective cell of the plurality of cells comprising battery connection wires connecting battery terminals of the respective cell to a cell balancing circuit, the method comprising:
 for each respective cell of the plurality of cells and for a contiguous group of two or more adjacent cells:
 controlling a pack current through the battery to be zero; 
 measuring a first cell voltage of the respective cell, or a first contiguous cell group voltage as a sum of respective cell voltages of the two or more adjacent cells in the contiguous group, with no pack current flowing through the battery; 
 controlling the pack current through the battery to be non-zero; 
 applying a balancing current to the respective cell or to each of the two or more adjacent cells in the contiguous group, equal to the pack current and flowing in an opposite direction to the pack current; 
 measuring a second cell voltage of the respective cell, or a second contiguous cell group voltage as a sum of respective cell voltages of the two or more adjacent cells in the contiguous group, with the balancing current applied to the respective cell or to each of the two or more adjacent cells in the contiguous group; 
 determining a total voltage drop across the battery connection wires of the respective cell or of the two or more adjacent cells in the contiguous group based on the difference between the first and second cell voltages; and 
 determining a total parasitic resistance of the battery connection wires of the respective cell or of the two or more adjacent cells in the contiguous group based on the total voltage drop and the balancing current; and 
 
 computing a respective parasitic resistance of respective battery connection wires in the battery based on the total parasitic resistances of the battery connection wires. 
 
     
     
       2. The method of  claim 1 , wherein computing the respective parasitic resistance of respective battery connection wires in the battery comprises solving a system of linear equations. 
     
     
       3. The method of  claim 1 , wherein the method is performed for each respective cell and for the contiguous group of cells in real-time. 
     
     
       4. The method of  claim 1 , wherein the method is performed for each respective cell and for the contiguous group of cells at different times. 
     
     
       5. The method of  claim 1 , wherein the contiguous group of cells comprises two adjacent cells. 
     
     
       6. The method of  claim 1 , wherein the contiguous group of cells comprises all of the plurality of cells. 
     
     
       7. The method of  claim 1 , further comprising:
 using the computed parasitic resistances of respective battery connection wires to estimate a battery state of the battery. 
 
     
     
       8. The method of  claim 1 , wherein:
 the cell voltages are measured using the battery connection wires, opposite to the battery terminals of the respective cells. 
 
     
     
       9. The method of  claim 1 , further comprising back-calculating actual cell terminal voltages based on the measured cell voltages and using the parasitic resistances when the balancing current flows. 
     
     
       10. The method of  claim 9 , further comprising estimating, using a battery state estimator, the battery cell states-of-charge and open-circuit voltage using the back-calculated actual cell terminal voltages. 
     
     
       11. The method of  claim 1 , further comprising:
 tracking the parasitic resistances of the battery connection wires over time and periodically comparing the parasitic resistances to a recorded nominal value; and 
 checking for a mismatch beyond a predetermined mismatch threshold to determine whether corrosion or connection faults have occurred. 
 
     
     
       12. The method of  claim 11 , further comprising:
 communicating with an external control unit in the battery management system when the mismatch exceeds the predetermined mismatch threshold. 
 
     
     
       13. The method of  claim 1  wherein the battery is a lithium-ion battery.